1. Field
Embodiments described herein relate generally to a readout circuit for sensing a cell current which flows through a memory cell, and a semiconductor storage device.
2. Background Art
A readout circuit in a conventional semiconductor storage device such as a flash memory includes, for example, a sense amplifier, which compares a cell current to a reference current, and outputs data “1” when the cell current is greater than the reference current and which outputs data “0” when the cell current is smaller than the reference current , for example.
The sense amplifier needs time for comparing the cell current to the reference current, and a comparison decision time required for data “1” is approximately equal to that required for data “0.”
In recent years, the cell current of the memory cell has become small (several tens of μA or less, or in the nA order in some cases) as the size of the memory cell shrinks. Therefore, the reference current used to read out data from the memory cell also needs to be a small current (a current which is at least smaller than a cell current of a memory cell in the on-state and greater than a cell current of a memory cell in the off-state).
If dispersion of the reference current becomes great, then designing the readout circuit (sense amplifier) becomes very difficult. And if the margin of the readout operation is insufficient, the readout circuit may conduct erroneaous readout.
Dispersion or variation of the reference current is observed with dispersion of characteristics of the MOS transistor generating a constant current, which becomes the reference current. Therefore, a circuit design which takes into consideration or compensates for this dispersion is demanded.
Conventionally, an ideal value of the reference current is set to a middle value between a cell current in the on-state of the memory cell and a cell current in the off-state thereof.
Since the cell current in the on-state of the memory cell is small as described above, however, it is difficult to secure the margin of the readout operation of the sense amplifier.